"It has been an incredibly tough couple of months for our entire family," said Middleton.
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Kate Middleton says she was diagnosed with cancer, is undergoing chemotherapy
The type of cancer has not been disclosed but Middleton said it was found after she underwent abdominal surgery in January.
"It was thought that my condition was non-cancerous. The surgery was successful. However, tests after the operation found cancer had been present," said Middleton.
Middleton says her medical team then advised preventative chemotherapy. ABC News spoke to Dr. Arif Kamal, an oncologist with the American Cancer Society about the importance of this treatment.
"What we recognize is that cancer shows up in millions and billions of cells that we can't just see with our normal eye. So we use chemotherapy to come and kind of clean up the rest. Make sure that there is nothing left behind," said Dr. Kamal.
MORE: Kate Middleton's cancer announcement likely timed to protect her kids, expert says
Dr. Laura Esserman, director of UCSF's Breast Cancer Center, hopes this announcement encourages the public to get screened.
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"If you are at high risk, are you at high risk for what type of cancer? And there you can adjust what you can do to reduce risk and to screen appropriately; if you are at risk for fasting-moving cancer you need to probably screen earlier and more frequently," said Dr. Esserman.
The Princess of Wales said her diagnosis was shocking.
"There is sometimes a myth that if you are young and if you are healthy, that you can't get cancer. Of course that is not true. We do know that if you happen to inherit a mutation and a particular gene your biggest period of risk is often in your 30s. So if you are BRCA1 or BRCA1 2 - it doesn't just come with breast cancer risk but ovarian cancer risk. For BRCA1 and 2, pancreatic cancer risk and for men prostate cancer risk," said Dr. Esserman.
Kate Middleton: A timeline of her cancer diagnosis, surgery and absence from public duties
At Crown and Crumpet in San Francisco, the Bay Area British community is hoping this announcement saves lives.
"Charles who was also in the hospital in January as well for prostate problem. He came out immediately and promoted the idea of men getting checked for prostate cancer which doesn't happen," said Christopher Dean, owner of Crown & Crumpet. "It is a worry. As far as I understand, they are both out of danger but who knows. We are all human."
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At the end of her announcement, Kate thanked the public for their support in recent weeks, adding that she and her family "now need some time, space and privacy while she completes the treatment."
Dr Esserman is encouraging women to sign up for the Wisdom Study, a free study that she leads to help understand cancer risks specifically for breast cancer.
To sign up, click here.